A sentence in future perfect continuous tense tells two things
a) An action of future (denoted by verb ) is not completed but going on.
b) Starting time of action /work or period of time during which action will be taking place is given. It shows that work will be completed in some future time.
1) 1st form of verb is used.
2) “ing” is added to the 1st form of verb.
3) Helping verb à Shall have been / will have been is used as.
a) shall have beenà I , we.
b) will have been à all other subjects.
4) Before Time “Since /for “is used (see the correct use of since/for). “Since” is used if ‘time of beginning of action’ is given and “for” is used if ‘period of time’ is given.
Affirmativeà Subjects +shall have been /will have been +verb + objects.
Negativeà Subjects + shall / will + not have been + verb +object.
Interrogativeà shall /will +subject +have been + verb + objects?
Negative + interrogative à shall/will +subjects + not have been + verb + object?
1) He will have been dancing for one hour.
2) She will not have been sewing since morning.
3) Shall I have been waiting for you 2 hours by 10 a.m.?
4) Will he not have been staying in hotel for two days?
5) Farmers will have been sowing wheat for two weeks in summer.
For practice see